Human Evolution

—By Karen Barss

The story of human evolution began in Africa about six million years ago and it describes the very long process that our ancestors went through to ultimately become modern humans. This process has been uncovered by studying fossils and understanding the underlying theory of evolution, and while new fossils are uncovered every decade revealing new chapters, scientists agree about the basic story.

What Is Evolution?

Evolution means the changes that occur in a population over time. In this definition, a “population” means a group of the same species that share a specific location and habitat. Evolutionary changes always occur on the genetic level. In other words, evolution is a process that results in changes that are passed on or inherited from generation to generation. It does not, for example, describe how people can change their muscle mass by lifting weights.

When successful, these genetic changes or adaptations, which happen when genes mutate and/or combine in different ways during reproduction, help organisms survive, reproduce, and raise offspring. Some individuals inherit characteristics that make them more successful at surviving and having babies. These advantageous characteristics tend to appear more frequently in the population (because those individuals with less advantageous characteristics are more likely to die without reproducing), and over time these changes become common throughout that population, ultimately leading to new species.

The Tree of Life

Biological evolution explains the way all living things evolved over billions of years from a single common ancestor. This concept is often illustrated by the so-called tree of life. Every branch on the tree represents a species. The fork separating one species from another represents the common ancestor that each pair of species shared. So ultimately, all life is interconnected, but any two species may be separated by millions or even billions of years of evolution.

Only a Theory?

Some people dismiss evolution as “just a theory.” Evolution is in fact a theory, a scientific theory. In everyday use, the word theory often means a guess or a rough idea: “My theory is…” “I have a theory about that.” But among scientists, the word has an entirely different meaning. In science, a theory is an overarching explanation used to describe some aspect of the natural world that is supported by overwhelming evidence.

Other scientific theories include cell theory, which says that all living things are made up of cells, and heliocentric theory, which says the earth revolves around the sun instead of the other way around.

The Relationship between Apes and Humans

Since scientists developed the ability to decode the genome and compare the genetic makeup of species, some people have been stunned to learn that about 98.5% of the genes in people and chimpanzees are identical. This finding means chimps are the closest living biological relatives to humans, but it does not mean that humans evolved from chimps. What it does indicate is that humans share a common ancestor with modern African apes (i.e., gorillas and chimpanzees), making us very, very distant cousins. We are therefore related to these other living primates, but we did not descend from them.

Modern humans differ from apes in many significant ways. Human brains are larger and more complex; people have elaborate forms of communication and culture; and people habitually walk upright, can manipulate very small objects, and can speak.

Our Common Ancestor

Most scientists believe our common ancestor existed 5 to 8 million years ago. Then two species broke off into separate lineages, one ultimately evolving into gorillas and chimps, the other evolving into early humans called hominids. In the millions of years that followed, at least a dozen different species of humanlike creatures have existed, reflected in the fossil discoveries of paleoanthropologists, although many of these species are close relatives but not actual ancestors of modern humans.

In fact, the fossil record does not represent a straight line of ancestry at all; many of these early hominids left no descendents and simply died out. Still others are most likely direct ancestors of modern humans orHomo sapiens. While scientists still do not know the total number of hominid species that existed, because new fossils are discovered every decade, the story of human evolution becomes clearer all the time.

What about the Missing Link?

The idea of a missing link has persisted, but it is not actually a scientific term. In the popular imagination, this missing link would be the fossil of our common ancestor. While scientists agree on the concept of a common ancestor, deciding which fossil represents that actual species is challenging if not impossible, given that the fossil record will never be 100% complete. Also, the word implies that evolution is a straight chain of events, when in fact the sequence of evolution is much more complicated.

The Fossil Record

Fossils are the remains or impressions of living things hardened in rock. All living organisms have not been preserved in the fossil record; in fact, most have not because very specific conditions must exist in order to create fossils. Even so, the fossil record provides a fairly good outline of human evolutionary history.

The earliest humans were found in Africa, which is where much of human evolution occurred. The fossils of these early hominids, which lived 2 to 6 million years ago, all come from that continent. Most scientists believe early humans migrated out of Africa into Asia between 2 million and 1.7 million years ago, entering Europe some time within the past 1 million years. What follows are some highlights of the early human species that have been identified by scientists to date.

Australopithecines

An African apelike species evolved probably around 6 million years ago with two skeletal characteristics that set it apart from apes: small canine teeth (the teeth on either side of the four front teeth) compared to the long canines found in almost all other primates, and, most importantly, bipedalism or walking on two legs as the primary mode of locomotion.

The name australopithecine means “southern ape,” in reference to South Africa where the first known fossils were found. Many more australopith fossils have been found in the Great Rift Valley in eastern Africa, in countries including Ethiopia, Tanzania, Kenya, and Chad.

The very early years of the transition from ape to human, from 6 million to 4 million years ago, is poorly documented in the fossil record, but those fossils that have been discovered document the most primitive combinations of ape and human features.

Fossils from different early australopith species that lived between 4 million and 2 million years ago show a variety of adaptations that mark this transition much more clearly. Among the genera that are included in early australopith species are Sahelanthropus, Orrorin, and Aripithecus; a species of the genus Kenyanthropus; and four species of the genus Australopithecus.

Probably the best-known australopith specimen is “Lucy,” the partial skeleton of a female discovered in 1974 in Hadar, Ethiopia. Lucy belongs to a species, Australopithicus afarensis, which thrived in eastern Africa between 3.9 million and 3 million years ago. Scientists have found several hundred A. afarensis fossils in Hadar. Lucy lived 3.2 million years ago.

Another very exciting A. afarensis site was discovered in northern Tanzania at Laetoli. In addition to fossilized bones of A. afarensis, researchers in 1978 discovered trails of bipedal human footprints preserved in hardened volcanic ash over 3 million years ago. The footprints provided irrefutable evidence that australopiths regularly walked upright.

By about 2.7 million years ago, so-called robust australopiths (in contrast to the earlier, gracile forms) had evolved, with wide molars and premolars and a facial structure that indicate that these robust australopiths chewed their food, primarily tough, fibrous plants, powerfully and for long periods. Several robust species have been identified, and the last robust australopiths died out about 1.4 million years ago.

The Genus Homo

The genus Homo first evolved at least 2.3 million to 2.5 million years ago. The most significant difference between members of this genus and australopiths, with which they overlapped, was their significantly larger brains (about 30 percent larger, though still small compared to modern humans).

Scientists divide the evolution of the modern human genus into three rough periods: early, middle, and late. Species of early Homo, among them Homo habilis, resembled australopiths in many distinct ways, but they had smaller teeth and jaws, more modern-looking feet, and hands capable of making tools. They probably lived from between 2.5 or 2.3 million and 1.6 million years ago.

The middle Homo species, including Homo erectus, evolved anatomically to be more similar to modern humans but their brains were relatively small (though bigger than australopiths). They probably overlapped with earlier Homo species, as they developed perhaps between 2 million and 1.8 million years ago. Homo erectus was a very successful species of the middle period; fossils have been found throughout Africa, Europe, and much of Asia, and the species may have survived for more than 1.5 million years.

The final transition, from the middle to late periods, happened about 200,000 years ago. Late Homo species, including Neanderthals and Homo sapiens, evolved large and complex brains, leading eventually to language, and developed culture as an increasingly important aspect of human life.

Homo sapiens

Scientists have dated the oldest known fossils with skeletal features typical of modern humans from 195,000 years ago. Early anatomically modern Homo sapiens fossils have come from sites in Sudan, Ethiopia, South Africa, and Israel. Many scientists have therefore concluded that modern Homo sapiens evolved in Africa and began spreading to other parts of the world 90,000 years ago or a little earlier, although whether, how, why, and when this happened is still in dispute. And it was not until about 40,000 years ago that anatomically modern humans,Homo sapiens sapiens, emerged. Since that time, human evolution has been primarily cultural as opposed to biological.

Putting Human Evolution in Perspective

Humans have existed for only a tiny fraction of Earth’s history. Scientists believe Earth itself is approximately 4.55 billion years old. The oldest known fossils are about 3.5 billion years old, although some scientists have discovered evidence that life may have begun nearly 4 billion years ago. Dinosaurs walked Earth between 230 and 65 million years ago. The oldest known humanlike fossil has been dated at 4.4 million years old, although another species, not yet confirmed as a hominid, has been dated at about 6 million years old. As mentioned earlier, scientists estimate that the earliest hominid species diverged from the ape lineage between 5 and 8 million years ago. And yet, the species to which we belong, Homo sapiens sapiens, is only about 40,000 years old.